Plasma torches are well-known tools for heating, welding, and cutting a variety of materials. U.S. Pat. No. 3,158,729 to Gross teaches a plasma torch design in which inert gas is ionized by a metal rod to which a high-frequency and high-voltage current source is connected. More often a plasma arc is established between two tubular electrodes by a high-voltage direct current source. The arc is often controlled or modified by a magnetic field created around the electrodes. Such a structure is taught by U.S. Pat. No. 5,132,511 to Labrot et al.
The inert gas which becomes ionized to form miasma usually flows through the torch at high velocity, and thus may carry the plasma "flame" to the object to be heated. In many applications a transferred arc configuration is used. In that situation, the sample must be conductive and is connected to the power source to become part of the anode of the torch. Then the plasma flows directly to the sample, thus heating it. If the sample is nonconductive, or if it is not feasible to connect the sample into the circuit, a nontransferred arc torch must be used. There, inert gas flow and an additional magnetic field are used to force the plasma "flame" to impinge on the sample. However, heating efficiency is often drastically reduced due to an electric charge that develops on the sample due to thermionic emissions and deflects the plasma from the sample.
Microgravity environments such as those found in orbiting satellites or space vehicles present special challenges and opportunities for plasma and other methods of sample heating. Under low gravity conditions, many materials can be purified and crystallized to yield samples with properties that cannot be obtained under normal gravity conditions. One of the advantages of the microgravity environment is that samples can be levitated and controlled readily. Normal gravity processing usually requires a container for the sample, but the ease of levitation under microgravity conditions allows containerless heat processing of samples. Containerless processing avoids both sample contamination and sample loss. Impurities picked up from the container may contaminate and affect final product quality. Also, valuable samples may be lost when sample adheres to the container during processing.
Therefore, there has been considerable attention to various methods of levitation and heating of samples for microgravity processing. U.S. Pat. No. 5,155,651 to Yoda et al. teaches an electrostatic levitator for use in microgravity experiments. U.S. Pat. No. 5,196,999 to Abe teaches an electrostatic levitation furnace in which the levitated sample is heated by electromagnetic radiation focused by an elliptical mirror. U.S. Pat. No. 4,979,182 to Lohoefer describes an electromagnetic levitator that uses four magnetic coils to levitate and heat an electrically conductive sample. U.S. Pat. No. 4,578,552 to Mortimer describes a single coil electromagnetic levitator/heater for an electrically conductive sample. U.S. Pat. No. 5,150,272 to Danley et al. teaches a plurality of separate coils for levitating and heating conductive samples.